In evolutionary theory, "fixation" refers to a situation in which every single individual within a population is homozygous for a particular allele. Thus, all current and future members of the population will display the phenotype conferred by the fixed allele.

For example, in a human population in which everyone has blue eyes, the allele for blue eye color is fixed. All the descendants of that population will continue to have blue eyes in the future, as long as no new individuals come into the population from elsewhere and as long as no mutations arise.